Threshold intensities of thermal radiation evoking sensations of warmth

Abstract

When human skin is stimulated with thermal radiation so that sensations of warmth are evoked, it is most sensitive to radiation with a peak wave-length of 35,u, according to . However, during an investigation of the effects of infra-red radiation on thermal comfort (Chrenko, 1953), discomfort was experienced when the temperature of the source of radiation was only 650 C, so that the skin was more sensitive to radiation with a peak wave-length longer than 3x5 It.Further experiments have been made to determine quantitative relations between the peak wave-length, i.e. the wave-length of maximum energy of thermal radiation, skin temperature, and the sensory threshold for warmth. METHODS ApparatusThe thermal stimulus was radiation emitted from a black body or full radiator. The inner core of the black body was a 30 cm diameter hollow sphere of ceramic material, with an aperture circular in shape and 10 cm in diameter, fitted to a hollow cylinder of the same diameter and length. Bricks of refractory material were used as thermal lagging, which covered the sphere so that the furnace assumed the shape of a cube with sides 60 cm in length.The furnace was mounted so that the longitudinal axis of the cylinder leading to the hollow sphere was horizontal. The furnace was electrically heated and its temperature was altered by varying the voltage of the electrical input. The maximum electrical loading of the furnace was 3 kW. The arrangements are shown in Fig. 1. Details of the thermocouple used to measure the furnace temperature are shown in Fig. 2. From the furnace temperature, the peak wave-length was calculated from Wien's equation, tf = (2892/A,, .) -273, where Amax. is the peak wave-length (,u), and tf is the temperature of the source of radiation (O C). The intensity of the radiation emitted by the furnace was measured with a calibrated thermopile with an output of 64-5 microvolts/milliwatt/square centimetre of incident radiant energy.A large water-cooled screen, with a circular aperture and shutter, was mounted vertically in front of the mouth of the furnace. The shutter was controlled by an electronic timing 1 Physiol. 173

Differences in the sensations of cold on the anterior torso of control and spinal-cord-transected individuals.

“…The differences that we observed between the sexes are an extension of the observations of differences in thresholds for men and for women reported by others (Chrenko, 1964; Fillingim & Maixner, 1995; Fillingim et al, 1999; Kenshalo, 1966; Meh & Denislic, 1994) These differences have been attributed to changes in circulating levels of sex steroids. In rats, ovarectomy and castration have distinct and different effects on the threshold for pain induced by formalin injections (Gaumond, Arsenault, & Marchand, 2002), and one can hypothesize that circulating steroids binding to membrane receptors in the central nervous system are important determinants of neural activity in sensory systems.…”

Perceived warmth and skin temperature as functions of the duration and level of thermal irradiation!

“…The much larger difference between the thresholds for "warmth" and for "change" may help to explain some of the variation among the measured values of the absolute threshold that have been reported from time to time. It is possible, for example, that the Ss examined by Hardy and Oppel (1937) were responding to "change," whereas those examined by Chrenko (1964) were responding to qualitative "warmth. "…”

“…Direct measurements of the absolute threshold for warmth vary from about 0.3 (Hardy & Oppel, 1937) to 1.6 millical/sec x em? (Chrenko, 1964), depending upon a variety of factors, such as the psychophysical method used, the area and location of stimulation, the skin temperature, and the "set" of the S. In order to learn how closely the inferred value. of 1.7 obtained by the iterative method corresponds to direct measurement of absolute sensitivity under reasonably comparable experimental conditions, we now turn to the measurements of the absolute threshold for "warmth" and "change.…”